1. Field of the Invention
The present general inventive concept relates to an illuminator to illuminate light emitted from a light source in a direction, and more particularly, to an illuminator which can reduce etendue of light emitted from a light emitting diode (LED) and simultaneously increase a flux density of the light.
2. Description of the Related Art
In general, an illuminator generates light and concentrates the generated light in a direction. The illuminator is employed in a liquid crystal display (LCD) device which cannot emit light by itself, or in an image projecting apparatus which generates an image using an image forming device, such as a digital micro-mirror device. For the illuminator to be employed in the image projecting apparatus, the illuminator needs to illuminate a collimated light.
The illuminator using an LED has many benefits as compared to a discharge lamp type illuminator. That is, the illuminator using an LED has a relatively long life span, can be made compact, can be operated at a low voltage, and has a light source that exhibits a fast switching speed. However, the illuminator also has drawbacks, such as a relatively large etendue and a relatively low flux density of light.
The performance of the image projecting apparatus employing an LED illuminator is determined by the etendue and brightness of the light source and an optical extent of the image forming device. To achieve the maximum optical efficiency in the image projecting apparatus, the etendue of the light source is required to be equal to or less than a limit etendue of the image forming device. Otherwise, a loss of light is generated. The etendue indicates the geometric property of an optical device related to the divergence of a beam and the size of a section of the beam. As the value of the etendue decreases, the flux density of light and brightness increase.
The etendue of a light source is determined by the size of a section of the beam at a target where the beam is focused and a solid angle of the beam. When an LED is employed as a light source, the chip size of the LED and the configuration of an optical system become major variables in determination of etendue.
As a solution to reduce the etendue value, U.S. Patent Publication No. US 2003/0128341A1, published on Jul. 10, 2003, and entitled “LED Illumination Engine Using a Reflector” (Inventor: Li, Kenneth K.) discloses a conventional LED illuminator configured as shown in FIG. 1.
Referring to FIG. 1, in a conventional LED illuminator, an LED 1 is approximately arranged at a focal point of a first reflective mirror 3 and emits light in a first wavelength range. The light emitted from the LED 1 is reflected by the first reflective mirror 3 to be a parallel beam and proceeds toward a second reflective mirror 5. The light is substantially focused on a focal point of the second reflective mirror 5.
A target 7 is positioned at the focal point of the second reflective mirror 5. The target 7 is a tapered light pipe having an incident end 7a at which all incident light is received and an exit end 7b from which the incident light exists. The size of a section of the light pipe gradually increases from the incident end 7a toward the exit end 7b. Since the light is incident on the incident end 7a of the light pipe at predetermined focusing angle, and exits from the exit end 7b with a reduced focusing angle, the solid angle determining the etendue decreases so that the etendue can be reduced.
However, when the conventional LED illuminator is configured as described above, since the LED 1 is a surface light source, not all of a light emitting portion of the LED 1 can be placed at the focal point of the first reflective mirror 3. Accordingly, some of the light illuminated from the LED 1 and reflected by the first and second reflective mirrors 3 and 5 is not incident on the light pipe, and a light efficiency is lowered. Also, since the tapered light pipe is needed, manufacturing costs are high. Furthermore, a large number of assembly steps are required for the accurate arrangement of the light pipe.